An effective one-pot hydrothermal method for in situ filling of multi-wall carbon nanotubes (CNT, diameter of 20-40 nm, length of 30-100 μm) with ultrafine ferroferric oxide (Fe3O4) nanoparticles (8-10 nm) has been demonstrated. The synthesized Fe3O4@CNT exhibited a mesoporous texture with a specific surface area of 109.4 m2 g-1. The loading of CNT, in terms of the weight ratio of Fe3O4 nanoparticles, can reach as high as 66.5 wt%. Compared to the conventional method of using a Al2O3 membrane as template to fill CNT with iron oxides nanoparticles, our strategy is facile, effective, low cost and easy to scale up to large scale production (∼1.42 g per one-pot). When evaluated for lithium storage at 1.0 C (1 C = 928 mA g-1), the mesoporous Fe3O4@CNT can retain at 358.9 mAh g-1 after 60 cycles. Even when cycled at high rate of 20 C, high capacity of 275.2 mAh g-1 could still be achieved. At high rate (10 C) and long life cycling (500 cycles), the cells still exhibit a good capacity of 137.5 mAhg-1.